This application seeks funds for continued time-resolved magnetic circular dichroism (TRMCD) studies of biological systems. The previous grant period demonstrated that it is possible to measure MCD spectra with nanosecond time resolution. It is now proposed that this technique be applied to several problems of biomedical importance. These fall into three areas: the study of intermediates which result from ligand photolysis from heme proteins and metalloporphyrins, the study of electron and proton transfer reactions in tautomerization reactions and in heme proteins, and the study of MCD spectra of molecular excited states. A few of these studies are aimed at further development of the TRMCD technique or development of theoretical frameworks needed to interpret MCD spectra. In these, artifacts which could interfere with TRMCD measurements will be investigated to be sure they are understood and can be eliminated under our experimental conditions. Also, simple systems will be studied which will be amenable to straightforward theoretical treatment so that these treatments can be optimized for understanding MCD spectral features. Most of the studies, however, will use the techniques already proven to be effective to study a variety of protein systems. These are aimed at understanding the nature of ligand, electron, or proton transfer reactions in proteins. These are critical processes which are responsible for a wide variety of processes necessary for life from transport of oxygen in the blood and muscles to production of species which can be used for energy sources by cells.